It might simply be a choice the developers made. Upscaling 1920x2160 has its benefits and perhaps with their rendering solutions looks better than upscaling 1800p. 1800p checkerboard might have also caused some frame rate dips, limited by gpu, while right now the pro is stable and seems to be limited rather by the cpu in some occasions.
What R* could have done (and what I kinda referred to in my original post as lack of enhancements) is offer a performance mode and one prioritizing picture quality. Having the vertical resolution native is great for wide screen open world games where you are always not far above the ground. Definitely a good choice to prioritize that.
What is surprising is that base XBox One is only 65% of the res of base ps4, or base ps4 having 1.56x the pixels of the base Xbox. If anything went 'wrong' during development it's there. Yet perhaps not that surprising as double the GPU rarely means double the pixel resolution. So the reverse is also true, you need to drop res a lot more to keep it running. If anything it suggests RDR2 development was done for the XBox One X and resolution turned down for the other consoles.
So the base Xbox running at 900p while PS4 runs at 1080p (like almost every single multiplat out there) is surprising to you, but the Xbox One X running at 100% higher resolution (with better frame rates) than the PS4 Pro is normal?
As I said, it might simply be by choice to keep the frame rate stable. 1800p might have made the frame rate more unstable, plus the temporal aliasing might have looked worse applying it for both axis.
XBox One X also has a faster CPU and much faster memory than ps4 pro.
PS4 has a slower cpu than base XBox One and eDRam can make a difference as well if used correctly.
Open world games are bound by CPU and memory speed a lot sooner than linear games. So if Pro uses the same hi-res textures as the X, the X has the advantage of the faster memory as well. Plus having more memory available also helps in keeping the frame rate stable.
So, nope, doesn't surprise me. 1920x2160 is equivalent to 1525p. It's better than 1440p, scales better than 1800p, and the 1.38x increase to 1800p could make the frame rate dips worse. What they could have offered is cinematic mode, 2.35:1 with black bars at a native 3840x1634 (that's still more than 1800p) and lock it at 24fps :) Most tvs support 24p judder free. Truly cinematic!